What other universities use STEP as an admissions test?
Cambridge has long had a tradition of setting its own admissions tests even before STEP was introduced in 1987. Written exams combined with usually two in-person interviews by the fellows (which were really tests) made the admissions process academically rigorous, on top of what one could produce for grades and recommendations. The admit rate was around 33% back then (people who didn’t have a real chance did not apply.)
Not clear whether a subset of US schools will decide to implement such an academically rigorous test to find the most academically able students. In-person interviews may be too costly with so many applicants.